Fiber Bragg gratings have found wide application for both telecommunications systems and sensor systems. In both cases, it is often desirable to precisely determine the returned optical frequency from a fiber Bragg grating. For example, fiber Bragg gratings can be used to sense a variety of physical parameters such as strain, temperature, displacement and acceleration. Each fiber Bragg grating sensor device can be configured such that the parameter of interest can be directly related to the optical frequency reflected by the fiber Bragg grating sensor.
A variety of interrogation techniques have been developed to accurately and precisely determine the returned frequency from a fiber Bragg grating. Several of these interrogation techniques rely on a scanning filter approach, such as a Fabry-Perot tunable filter or an acousto-optical device. These detection schemes typically rely on the characteristics of the filter to remain constant; however, the response of such filters typically change with changes in environmental conditions such as temperature, pressure or aging of the filter. Therefore, it is necessary to use a correction or referencing method to measure and back-out any environmental effects on the filters and eliminate possible errors associated with the changing characteristics of the filter. Additionally, since the changes may not be constant across the full bandwidth or response of the filter, the referencing method should cover the entire bandwidth of the filter.
One method to characterize a filter across its entire bandwidth for changing environmental conditions is to use a fixed wavelength referencing array of stabilized fiber Bragg gratings. The array is selected to have fiber Bragg gratings with selected frequencies covering the bandwidth of the filter. The benefits of using such a referencing array include the ability to space the gratings at arbitrary distances along a single or multiple fibers or co-locating all the gratings in a single spot on a fiber. Additionally, the grating can be written to provide a custom set of reference frequencies, depending upon the application.
It is desirable to control such an array to provide a fixed (constant) response. However, since gratings are sensitive to temperature and strain, the use of gratings as a reference requires precise control of the environmental conditions of the array. The strain response of a fiber Bragg grating is given by the following equation: EQU .DELTA..lambda.=(0.78).times..lambda..sub.R .times..DELTA..epsilon.
Where .lambda..sub.R is the reference wavelength of the fiber Bragg grating in an unstrained condition and .DELTA..epsilon. is the change in strain in the grating. The temperature response of the fiber Bragg grating is given by the following equation: EQU .DELTA..lambda.=(7*10.sup.-6).times..lambda..sub.R .times..DELTA.T
Where .DELTA.T is the change in temperature of the grating.
One method of providing reference optical signals is to provide passive temperature compensation for a reference array of fiber Bragg gratings. For example, U.S. Pat. No. 5,757,540 to Judkins, et al. discloses the packaging of a fiber grating in a single material such that a temperature-induced shift in the reference frequency is compensated by a corresponding strain-induced shift arising from the packaging material. Similarly, International Patent Application WO 98/27446 to Miller discloses temperature compensated fiber Bragg gratings wherein the fiber Bragg gratings are mounted on a structure that comprises two plates made of materials having dissimilar temperature coefficients of expansion and bonded together. The structure bends with changes in temperature to cause a strain in the fiber that compensates for temperature induced wavelength shifts.
The forgoing methods of passive temperature compensation are sufficient where highly stable and precise optical reference signals are not necessary. However, such passive temperature compensation methods are not suitable for certain applications, such as sensor applications, where precise and stable optical reference signals are necessary. There therefore exists a need for a method and apparatus for providing one or more stable and precise optical reference signals.